In standard loudspeaker systems electro dynamic transducers can fail due to either excessive voice coil excursion, causing mechanical clipping, or by overheating of the voice coil causing degradation of the materials.
A system in FIG. 1, as disclosed in U.S. Pat. No. 5,528,695, uses a prediction of the displacement in order to limit the incoming signal in a feed-forward manner with the purpose of limiting the displacement caused by this applied signal. Limitations of this system mean that the non-linear behaviour of the system is not included in the prediction of the instantaneous displacement xd(n) and hence a suboptimal control may be obtained if the system is designed to operate at the mechanical limit or beyond for a significant period of time. Furthermore it describes multiple feedback configurations which do not guaranties a limitation that is fast enough, hence a suboptimal threshold must be applied limiting the performance of the system.
Document WO 2005/091672 describes principles for limiting the displacement by filtering the incoming voltage related signal by a transducer response related shelving filter response. Shortcomings of this system relates to the fact that no explicit look-a-head time is included for the system to limit the signal. This implies that an instantaneous peak in displacement may parse through the system without proper attenuation if the filtering is not done with enough headroom leading to suboptimal performance in situations where maximum output is required.
Document WO 01/03466, describes a system for multiband protection based on a power measurement and estimate. This system does not directly apply the physical measure of displacement in order to limits this quantity, but do so in an implicit overall consideration of the system capabilities leading to suboptimal control.